Noble gas ion lasers were reduced to practice twenty years ago, and since that time the continuous wave (CW) argon and krypton types have dominated all other laser types in applications requiring high average power in the visible spectrum. The continuing search for new laser media has turned up nothing to replace them. The incentive to replace them has been strong, particularly in high power applications, because they require that kilowatts of waste heat be removed from their working environment. By definition their discharge tubes must run continuously, because their critical gas density is established and stabilized from a higher cold-fill pressure, in a slow process of thermal driveout that can take minutes to reach equilibrium.
The reader can best appreciate the practical limitations of the continuous laser by considering an analogy between it and an electric motor. Imagine an electric motor that draws full rated current for any shaft speed selected, regardless of whether there is load on the shaft. In the analogy shaft speed represents laser power setting. This hypothetical motor may be doing no work at all, yet has to draw the same current it would if fully loaded at the same speed. This burden might be overlooked if the motor was reasonably efficient, but it is not. The heat would drive people from the workplace, so it must be removed, by putting it into water that goes down a drain or into air that is vented outdoors. Imagine how many motors having such a requirement would find application. This analogy represents the existing requirement of continuous wave ion lasers producing 1 watt or more of output. It is clear that only the unique properties of laser light, and the lack of alternative sources, can explain the numbers of such lasers in service.
Most of the work of these lasers, including the medical treatments, is performed without needing continuous output. In fact, it would be impossible to treat with a laser continuously. A laser surgeon uses far more off-time than on-time.
A second species of ion laser, the REPETITIVELY-PULSED type, has been a practical, but limited, alternative to the CW ion laser. Its characteristic bore dimensions are larger and its pulse currents are higher. Its pulse power may be ten times that of the equivalent continuous laser, so it needs only a burst of pulses at ten per cent duty factor to deliver the same average power. Furthermore, it can be air cooled if, between the high-duty bursts, it can idle at low enough duty factor to average the heat dissipation to a level the room space will tolerate. Of course the tube design must be such that the gas's thermal expansion during increases of duty factor does not thin it so much as to destabilize the output or cause arc starvation (current interruption). Starting with a higher fill pressure, in anticipation of gas thinning, is not the answer because then the tube would lose output stability at low duty factors due to a dense-gas effect known as radiation trapping.
To the present time, there has been one repetitively-pulsed gas laser that can tolerate changes of duty factor. It is the novel design patented in 1971 by Michael R. Smith (U.S. Pat. No. 3,626,325), in which the discharge column gas density is kept within critical limits, as the average demand on the tube varies, by surrounding the bore with an envelope containing near-vacuum gas, and causing radiative transfer to be the main method of heat disposal. Since radiative transfer varies as the fourth power of the absolute temperature large changes of power input result in relatively small changes of bore temperature. This moderating effect reduces thermal drive-out of the gas and keeps the column density within acceptable limits over a useful duty-factor range. The Smith laser has functioned well in a number of applications that did not require pulses longer than about 150 microseconds.
The pulsed gas laser tubes that preceded the Smith laser were convectively cooled, so their heat transfer was a first power function of temperature. They could barely double their duty factor without going out of gas-density range, and they delivered only tens of milliwatts of average power.
In what is known as "burst" mode the Smith laser has been able to deliver over two watts of average power, more than enough to perform treatments of the eye, without needing heat disposal methods that continuous lasers in the same application require. During the individual treatment exposures a rapid mixing of pulses and off-times is equivalent to continuous delivery of energy if the pulsing is faster than the thermal response time of the tissue and if the same energy is delivered in the same time interval. But the Smith-type laser offers a versatility of treatment not possible with continuous lasers. If the surgeon wishes to control the depth and spread of thermal tissue effects he can do so by varying the pulse duration and repetition rate. If he makes the pulses very long and the spot size very small the characteristically high pulse power yields enough energy density within the time of individual pulses to enhance evaporative effects in tissue-perforating processes such as are used for glaucoma treatment. Thermal damage to surrounding tissues is reduced, and it could be reduced further if pulses longer than 150 microseconds were possible.
In all types of applications there is another important advantage of the Smith-type laser. It can maintain nearly optimum efficiency at low power delivery as well as high, because of its pulsed-current excitation. It lowers its delivered power by lowering either the frequency or the duration of its pulses, while still driving each pulse with an efficient, high-level current. A CW laser can only reduce its power by reducing its current, and its efficiency is steeply reduced as it does so. Furthermore, the method of lowering the current in CW lasers is to drop some of the supply voltage across series-connected pass transistors, a dissipative process that adds to the power waste and to the water cooling needed. Even the transistors are water cooled. The Smith-type lasers can be turned on and off by switching-mode transistors, so their power control is non-dissipative.